Looking up the capacity online without knowing the engine size is unreliable and could lead to overfilling or underfilling, both of which can damage your engine.
If you don't have the owner's manual, you can often find a PDF version online by searching for "[Your Buick Electra Engine Size] owner's manual". You'll need to find the engine size (e.g., 3.8L, 4.1L, etc.) usually found on a sticker under the hood or on the driver's side doorjamb.